FIG. 8 is a conceptual view showing a configuration of a typical breaker provided with a vacuum interrupter 35. A breaker 30 is provided with an insulating frame 34 housing therein the vacuum interrupter 35 and installed on a carriage 31. The vacuum interrupter 35 has a fixed-side connection conductor 36 connected to a fixed electrode bar, a flexible conductor 37 connected to a movable electrode bar, and a movable-side connection conductor 38. The fixed-side connection conductor 36 and the movable-side connection conductor 38 are introduced to an outside of the insulating frame 34. Installed at a front of the carriage 31 are a face plate 32 and an operation mechanism 33.
A vacuum interrupter adopted in such a breaker includes a bottomed cylindrical vacuum container made of an insulating material, such as a glass material and a ceramic material, and having a highly evacuated interior, electrode bars respectively provided to both end portions of the vacuum container, spiral-ring-shaped coil electrodes provided to opposing end portions of the respective electrode bars, reinforcing members reinforcing contacts, and disc-shaped contacts. A current is passed or interrupted as the both contacts, that is, a fixed contact and a movable contact, are brought into contact with or spaced apart from each other by moving one electrode bar in an axial direction. The coil electrodes referred to herein mean coil electrodes provided with plural arc-shaped coil portions installed to the both contacts on a rear surface side in a divided manner in a circumferential direction along peripheries of the contacts and having an arm portion in the axial direction at one end of a coil and a protruding portion connected to the contacts at the other end so as to generate an axial magnetic field in a direction in which a fixed contact and a movable contact as a main electrode come close to and move apart from each other.
In the vacuum interrupter as above, the coil electrodes generate a field in the axial direction as a current is passed, and a current density is lowered for the contact surface by diffusing an arc between the contacts inevitably generated when the current is interrupted over the contact surfaces while trapping the arc within diameters of the contacts. Accordingly, the contact material outperforms in interruption capability and a current is interrupted.
In the vacuum interrupter that enhances an interruption capability by generating an axial magnetic field, an eddy current is induced at the disc-shaped contact and there is a problem that a field generated by the eddy current weakens the axial magnetic field by the coil electrode. It is known to provide radial slits to the contact to avoid the eddy current. The slits penetrating through the contact, particularly in a vacuum interrupter used in a class of high rated voltage, may possibly become a weak point portion in capability for withstanding high voltage between the opposing contacts. Hence, it is also known to provide a radial groove not penetrating through the contact to the contact on the side of the coil electrode.
When an arc is ignitiond between the fixed contact and the movable contact of the vacuum interrupter when a current is interrupted, a current (arc current) flows through the fixed contact side, that is, through the fixed-side connection conductor connected to the fixed electrode bar and through the movable contact side, that is, through the movable-side connection conductor connected to the movable electrode bar, and an electromagnetic force is generated. The electromagnetic force drives the arc in a direction in which the electromagnetic force acts and thereby moves the arc from the ignition position. As the arc moves, a large part of the current passes through a connection portion at a nearest position in the direction in which the electromagnetic force acts and then flows into the coil portion of this connection portion. In short, the current does not flow homogeneously to the respective coil portions forming the coil electrode. Accordingly, a field generated in a coil portion in which a larger amount of the current (a large part of the current) flows becomes stronger than fields generated in the other coil portions. On the contrary, because an arc has a characteristic of spreading in a region where the axial magnetic field strength is strong at or higher than a given value, an arc diffuses along a region (extending region) extending along the circumferential direction of a coil portion through which a larger amount of the current flows.
However, in a normal coil electrode, plural coil portions are merely provided in a divided manner at equal length. Consequently, an arc intensifies in a relatively narrow area along the extending region of one of the equally divided coil portions. This raises a problem that the main electrode (contact) is damaged or consumed significantly due to local overheating and an interruption capability is deteriorated by overheating. It is therefore known to make a coil portion at a particular point longer than the other coil portions instead of the coil portions having an equally divided length.